What is the quality? In dictionary Dalya following definition contains: quality - property or an accessory, everything, that makes essence of the person or a thing. What to do, if we need to estimate visual quality, for example, video compressed by the codec? It is possible to understand quality of video as an average estimation of opinions of the people, looking it is video. This parameter founders of systems of processing of video finally wish to improve, therefore it would be desirable to be able for it to estimate numerically. There are two approaches to the decision of this problem: a subjective and objective estimation of quality of video. In the first part of this clause we shall tell about these approaches, and in the second we shall result results of the subjective comparison of modern videocodecs lead by us. Objective testing It is possible to estimate quality of video by means of any formula or algorithm, for example, PSNR, VQM or SSIM (See). The main advantage cd drive notebook rom of this approach in an opportunity to automate process of testing, that Allows to measure quality of processing of a videosystem at a plenty of various adjustments and test Video or to measure quality in real time. Also as a result of measurements turn out exact and reproduced Data. A minus of this approach that automatic metricss can incorrectly reflect subjectively perceived Quality. It can result, for example, in an incorrect conclusion about the superiority of one codec over another. Subjective testing the Alternative way to receive an estimation of quality of video - to lead subjective testing. The idea of this method in receiving an estimation of quality is direct from the spectators spending an estimation Video. The similar approach in an estimation of quality of a sound is used already long time. For example, at a forum On a regular basis pass subjective testings audiocodecs. What is necessary to lead subjective testing? To choose videosequences for testing. Video about 8- seconds, that is usually used In 19 the first version of recommendations ITU-R BT.5 " Methodology for the subjective has cd drive notebook rom been published assessment of the quality of television pictures ". These recommendations contain exhaustive description of that, As it is necessary to solve all the set forth above questions. Since then subjective testings were spent many, (Video Quality Experts Group). In spite of the fact that subjective testings were repeatedly spent by the various organizations, up to Recent time in an easy approach was not stable programs for carrying out of the testing, calculated For work with personal computers. It became the reason of development in which some methods of subjective comparison and the analysis are realized Results. Methods of subjective testing the Method of subjective testing is a combination of ways of demonstration of sequences, gathering of opinions Experts and processing of results. We shall consider on an example of comparison of videocodecs procedure of testing by method SAMVIQ recently developed In EBU (European Broadcasting Union), realization MSU Perceptual Video Quality tool. This method was used In Subjective Comparison of Modern Videocodecs. The scheme of method SAMVIQ Stages of testing: 1. The expert enters the name (any unique sequence of symbols). 2. The test cd drive notebook rom for colour perception (standard tables Ishihary' are used). 3. For each test sequence: reference (initial) video Is shown. Until there are not seen compressed variants of it video, the expert chooses the next variant Video, looks it and assesss. The estimation for film belongs to a piece from up to 100, than Above, the better. The estimation of already seen variants of sequence can be changed in If all variants of video are seen, the expert can pass to following test sequence. Different variants of the compressed sequence are hidden behind letter designations, therefore the expert Does not know, it estimates what codec at present. Reference video is accessible obviously, also it It is hidden under one of letter designations and it is estimated on the same level with the compressed videosequences. For what such complexities are necessary? There Are some problems which should solve techniques subjective Testings. First of them � creation at all experts of the general rating scale, that is that The estimation "well" meant for different experts approximately the same. It is reached by means of reception Under the name �anchoring�: during the test it is shown as cd drive notebook rom video with the highest quality (� high anchor �, Should associate at all experts with the maximal estimation), and with the lowest (� low anchor �, should To associate with the minimal estimation). One more problem � minimization of effect of memory, influence of sequence of display of video on Estimations of experts. In some test methods this problem is solved by means of demonstration referensnogo (Initial) video together with each processed videosequence. In method SAMVIQ which we used At the comparison, the first problem is solved by means of obviously accessible and latent reference video, and the second � By means of more flexible, than in other methods, procedures of an estimation (the expert can reconsider video And to change the estimations). At any test method the set of foreign factors can affect results of subjective testing. It is necessary, that all experts have been instructed about a way of passage of testing, in was obespechenno sufficient illumination, testing should not tire experts. Change results a little can everything, everything, from a floor of experts up to their trades and time of carrying out of testing. It is interesting, that in comparison with all other factors of the characteristic of the monitor cd drive notebook rom (the sanction, LCD/CRT, etc.) do not render significant influence on results (see M.Pinson, S.Wolf, � The Impact of Monitor Resolution and Type on Subjective Video Quality Testing � NTIA TM-04-412). Processing of results the Basic results turn out after simple averaging estimations on experts. The received estimation refers to MOS (Mean Opinion Score). Also for an estimation of disorder of opinions the confidential interval (an interval is usually resulted, In which with the set probability there is a real average opinion). There are the techniques allowing To exclude the experts, giving results astable and strongly differing averages. Subjective comparison of modern videocodecs In the end of 20 our laboratory has been lead subjective testing videocodecs. Problems Testings were subjective comparison of new versions of popular codecs, comparison of results With data of objective metricss and working off of technology of subjective testing. In this clause it is resulted x2 The Codec with open initial code Core svn-352M by Sharktooth WMV Bit rate 7000 bps, 10485 bps cd drive notebook rom Other parameters of codecs remained without changes. Test videos: The name Length [staff] Length [seconds] the Sanction the Source Matrix sc. 2 staff x 4 MPEG2 Matrix sc. 2 staff x 4 MPEG2 (DVD) sequences from films � Terminator � and "Matrix" Were used: two an average and two with very Fast movement. As a method of subjective testing it was used SAMVIQ, described above. Subjective testing Passed in current of three days. In total experts have taken part in testing. Three types were used Monitors: x � CRT Dell, x � CRT Samsung and x � LCD Samsung. Following schedules show results of testing on one of sequences. On an axis of ordinates average subjective opinion (MOS, the above, the better) and % a confidential interval, that is for the given volume of sample real value MOS is in the specified range with probability 0.95, Ref � initial video, on an axis absciss � the codec and bitreyt with which video was compressed. MOS for sequence Battle �Battle� � sequence with very strong movement. From the schedule it is visible, that the codec x2 with bitreytom 6 kbps has been estimated the same cd drive notebook rom as and WMV with bitreytom 10 kbps. It is interesting, that initial video (received With DVD) has not received the maximal mark 1 though possessed the best quality � experts saw In it artefacts. MOS for sequence Rancho In sequence �Rancho� movement is much weaker, many codecs have coped it practically equally � It was more difficult to experts to distinguish them, the disorder of estimations has increased. Nevertheless, the superiority x2 all Equally noticeably. On the following schedule it is possible to see values MOS average on all sequences. It is obvious, that in average opinion of experts the codec x2 considerably surpasses all other tested Codecs. The low result of codec XviD is consequence of that in the decoder of this version debloking not It is included by default. It has not been included in connection with the accepted policy of non-interference in thin (for usual The user) adjustments of the codec. Conclusions Subjective comparison � a unique output if you need to estimate real quality of video. cd drive notebook rom Exists Set of details at carrying out of comparison to which it is necessary to pay attention, but at observance The certain rules correct application of techniques of testing can yield reliable and valuable results. The full text of comparison with the analysis of subjective results and gauging of objective metricss is to the address of Oleg Peters, MSU Graphics Other discussions in conference:

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